Cytokinesis is the final process of the cell cycle in which a cell cleaves in two. In animal and fungal cells a contractile ring mediates cytokinesis; conserved proteins make up this ring and are needed for its assembly and function. We propose complementary biochemical and genetic approaches, utilizing the powerful experimental tools of S. cerevisiae, to investigate the mechanism of the contractile ring. First, we propose purification of functional contractile rings from S. cerevisiae. This purification should identify additional components of the contractile ring, serve as a starting point for detailed study of its ultrastructural organization, and provide an in vitro functional assay to investigate the importance of ring factors. In the event that proof of principle experiments demonstrate that the former, rather ambitious, approach is not currently feasible, an alternate genetic approach will be undertaken. This approach takes advantage of the experimental tractability of S. cerevisiae to identify proteins involved in cytokinesis likely to be missed in screens in other organisms. Genes identified in this screen may encode for ring constituents, ring disassembly factors, regulators of ring constriction, and factors mediating attachment of the ring to the plasma membrane. This study should have lasting impact on our understanding of this fundamental biological process.